Multiple Choice
Which of the following lists the three fundamental rules that govern the filling of atomic orbitals by electrons?
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10. Periodic Properties of the Elements
The Electron Configuration
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Which of the following is the correct electron configuration for cesium (Cs)?
A
1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^6 5s^2 4d^{10} 5p^6 6p^1
B
1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^6 5s^2 4d^{10} 5p^6 6s^1
C
1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^6 5s^2 4d^{10} 5p^6 4f^1
D
1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^6 5s^2 4d^{10} 5p^6 5d^1
Verified step by step guidance1
Identify the atomic number of cesium (Cs), which is 55. This means cesium has 55 electrons to be placed in orbitals according to the Aufbau principle.
Recall the order of filling orbitals based on increasing energy levels: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, and so on.
Fill the orbitals with electrons following the Aufbau principle, Hund's rule, and the Pauli exclusion principle until all 55 electrons are assigned.
Note that after filling the 5p orbital (which accounts for 46 electrons), the next electron goes into the 6s orbital, not 6p, 4f, or 5d, because 6s has lower energy than those orbitals.
Write the electron configuration for cesium as: $1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^6 5s^2 4d^{10} 5p^6 6s^1$, reflecting the correct placement of the 55th electron.
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